1. Field of the Invention
The present invention relates to a presensitized plate, more particularly to a positive working presensitized plate having a thermosensitive layer, of which solubility to an alkali aqueous solution increases (namely, becomes alkali-soluble) by heating due to photothermal conversion by a laser beam or the like, which is excellent in both a quality that is less likely to scratch (damage resistance) and sensitivity (development performance), excellent in both a quality less likeliness to get dirty (scum resistance) and press life at printing. In addition, the present invention relates to a positive working sensitized plate having a thermosensitive layer containing a novolac resin, which is excellent in press life after burning-in processing.
2. Description of the Related Arts
With development of image formation technology in recent years, it has come to be possible to perform direct plate making by scanning narrow focused laser beams on the printing plate to form a manuscript of letters, images and the like directly on the plate without using a film manuscript.
In a presensitized plate of a so-called thermal positive working type for causing photothermal conversion in a recording layer (thermosensitive layer) by laser beam irradiation to make the recording layer alkali soluble, and thus forming a positive image, a subtle change in interaction of binder molecules contained in the recording layer by laser beam exposure is utilized as an image forming principle. Accordingly, a difference in ON/OFF levels of alkaline solubility between exposed and unexposed portions is reduced. Therefore, for the purpose of obtaining clear discrimination to be put to practical use, use has been made of means for forming a recording layer structure by providing a surface slightly soluble layer in developer as an uppermost layer of the recording layer, and suppressing developer solubility of the unexposed portion.
However, when the surface slightly soluble layer is damaged for some reason, even a portion intended as an image area is made easily soluble in the developer. In other words, a printing plate produced is damaged very easily from a practical standpoint. Thus, scratch-shaped non-image portion is brought about by subtle contacts such as clashing in handling of the printing plate, subtle rubbing of an interleaving sheet, contact of fingers with the plate surface or the like. Accordingly, handling of the printing plate is very difficult in printing plate work under the present circumstance. In order to improve the damage resistance as described above, efforts have been made by providing a layer made of a fluorochemical surfactant or a wax agent on a surface of a recording layer so as to decrease coefficient of friction. However, these countermeasures have not sufficiently overcome the problem.
Meanwhile, a study has also been made for improving development performance in order to increase the discrimination. For this purpose, an attempt is made to provide a hydrophilic layer prepared by performing silicate treatment or an alkali soluble undercoat (layer becoming soluble in alkali) between recording layer and support. In accordance with these methods, though fairly high development performance is ensured and a development latitude at a practical level is obtained, adhesion between the recording layer and the support is decreased. In addition, when the profile of the support surface is made flat so as to remove deep concave portions on the support surface that causes residential layer for scum resistance improvement, press life thereof decreases so greatly that the presensitized plate becomes practically unusable. Therefore, a presensitized plate with a satisfactory performance in terms of ease in printing, that is, excellent in press life and also in scum resistance, has not been realized.
Meanwhile, in order to prepare the surface slightly soluble layer as described above, a typical method is to cause the thermosensitive layer to contain an acrylic resin with a novolac resin. However, a presensitized plate with a thermosensitive layer containing many acrylic resins is difficult to harden even performing after-heat processing (hereafter, referred to as a “burning-in processing”) after development, and thus press life thereof is not greatly improved.
Meanwhile, a thermosensitive layer mainly composed of novolac resins is cited as a thermosensitive layer of which press life is greatly improved by burning-in processing. However, such a presensitized plate with a thermosensitive layer mainly composed of novolac resins has not sufficiently formed a surface slightly soluble layer. Accordingly, when a sensitivity of the thermosensitive layer is designed such that an exposed area is developed, a developer is likely to be permeated also into an unexposed area. As a result, adhesion between the thermosensitive layer or an intermediate layer such as the undercoat layer (hereafter referred to as “thermosensitive layer or the like”) and the support decreases, thereby decreasing press life thereof. Namely, it was difficult to achieve both development performance and press life thereof.
Particularly, when an area with sharp grains of a support surface where an ink is likely to remain is reduced in order to improve scum resistance at the time of printing, the thermosensitive layer or the like is also unlikely to spread. Further, when the thermosensitive layer is hardened by burning-in processing, the thus hardened thermosensitive layer does not spread on the support. Therefore, substantially no effect of improving press life due to burning-in processing can be obtained. Namely, it is difficult to achieve both scum resistance and press life after burning-in processing.